1. Field of the Invention
This invention relates to control systems for gas kilns which automatically control the firing of the kiln both during the heating period and the cooling period.
2. Description of the Prior Art
The production of ceramic products from clay normally requires constant supervision of a kiln over an extended period in order to make sure that the clay being baked in the kiln is properly matured. The maturation point of the clay is very critical since under-mature ceramic products tend to be brittle while over-mature clay bodies will melt in the kiln. Due to the fact that both temperature and kiln chamber environment affect the maturation of clay, it is very difficult to determine the exact point of critical maturation. For this reason pyrometric piles or clay cones are used to indicate the point at which the clay is mature enough to discontinue the baking. The pyrometric pile has characteristics similar to the clay being baked and under increasing temperature it will gradually soften, sag, and finally prostrate itself to indicate ideal maturation. Normally this process is monitored by the kiln operator through a small orifice in the door of the kiln.
Certain pyrometric switches have been used in the past in order to automatically turn the kiln off when the maturation of the clay has been reached. Included among these is U.S. Pat. No. 2,675,600, issued Apr. 20, 1954, to Dawson. The Dawson patent teaches the use of a rod pivotally attached toward the center thereof with one end of the rod resting upon a cone. When the cone sags, the rod pivots lifting the opposite end thereof which actuates the switch to turn the kiln off. U.S. Pat. No. 3,287,530, issued Nov. 22, 1966, to Dawson, discloses a similar type pyrometrically actuated switch in which the pyrometric cone sags causing a release claw fastened to an actuating rod to move out of engagement from a moveable weight. The weight falls downwardly and a stud protruding therefrom releases a spring biased plunger having moveable conductor means or straps, thereby creating a circuit discontinuity and hence an interruption of power flow to the kiln. U.S. Pat. No. 3,838,377, issued Sept. 24, 1974, to Vukovich, shows a pyrometric member having means, such as a uniform transverse cross section or a notch in which the rod of a switch actuating mechanism may rest, to accurately regulate the amount of heat required to cause deformation of the member.
Certain control systems for controlling the temperature in kilns, furnaces, and other similar devices, have been used also. For instance, U.S. Pat. No. 2,505,041, issued Apr. 25, 1950, to Gorsuch, shows a drying apparatus having operation sequence control. The Gorsuch device uses a series of cam operated switches controlled by a motor to actuate certain heating coils in the tumbler motor of a conventional home drying apparatus. U.S. Pat. No. 2,977,452, issued Mar. 28, 1961, to Weber et al, shows a capacitance type temperature control for automatically controlling temperature within an enclosure. U.S. Pat. No. 3,901,437, issued Aug. 26, 1975, to Harkins, shows a programmed temperature control system for a furnace or kiln. The Harkins device uses a thermocouple position within the kiln to provide a signal indicative of the temperature of the kiln. A slidewire potentiometer provides a command signal indicating a preselected maximum temperature to be reached by the kiln, and a ramp generator provides a variable voltage, the contour of which is to be followed by the kiln when progressing from a low temperature to the maximum preselected temperature.